(a) Field of the Invention
The present invention relates to an organic electroluminescent (EL) device having a plurality of light emitting pixels arranged in a matrix and a method for fabricating the same, especially to the method of fabricating the organic EL device having a plurality of minute electrode lines formed by a vacuum evaporation process using a patterning mask.
(b) Description of the Related Art
An organic EL device utilizes a phenomenon in which a hole injected into an anode and an electron injected into a cathode are recombined in a light emitting layer to emit light through an excited state. Various device configurations of the organic EL devices have been investigated based on the properties of the light emitting materials of the light emitting layer. The device is basically obtained by sandwiching the organic light emitting layer emitting strong fluorescence by the anode and the cathode.
For attaining the higher light emitting efficiency and the stable operation, a charge injecting and transporting layer such as a hole injecting and transporting layer and an electron injecting and transporting layer or an interface layer is mounted or a guest molecule is doped into the organic light emitting layer. For the purpose of improving the light emitting efficiency and the lifetime, the material for the electrode, especially the cathode is investigated. The improvements of the structure and the material of the device provide the performance for the practical use though not completely satisfactorily.
Prototype full-color organic EL display units are reported as application of the organic EL device. Examples of the schemes for generating three colors in an EL display unit includes a separate-three-color emitting system in which the light emitting devices for the respective colors are mounted parallel on a substrate, a CCM system in which a color conversion layer is mounted on a light emitting surface and a blue light emitting device is used as an EL light emitting source, and a color filter system in which full-color display unit is attained by using a color filter. Each prototype color display unit is fabricated by using one of the systems, and among the three systems, the separate-three-color emitting system is most efficient because the structure is simple and the light emitting is effectively utilized.
A color organic EL display unit using a 320xc3x97240 pixel array and having a diagonal size of 5.7 inch is described in the NEC Technical Journal Vol.51, No.10, pp28-32 (1998) as the display unit using the separate-three-color emitting system. During the fabrication of the display unit, the electrode (cathode) lines are deposited through a metal mask pattern having 240 slits (lines). The pixel size of the display unit is 0.36 mmxc3x970.36 mm, and the sub-pixel size is 0.12 mm (120 xcexcm). A full-color organic EL display unit having a diagonal size of 5.2 inch and a 320xc3x97240 pixel is described (Extended Abstracts of 9th International Workshop on Inorganic and Organic Electroluminescence, September 14-17, pp137-140 (1998)). The cathode lines of the display unit are deposited by vacuum evaporation by utilizing the blocking of a partition wall having a reversed taper and made of resist. The pixel size of the display unit is 0.33 mm (330 xcexcm), and the sub-pixel size is 0.12 mm (120 xcexcm).
As described above, the organic EL device of the separate-three-color emitting system possibly provides the simple structure with the higher light emitting efficiency. However, in the formation of the conventional prototype electrode (cathode) lines by using a metal mask having apertures on the lines, the minute cathode lines can be hardly formed because the warp of the mask is generated due to the radiation heat during the formation of the cathode, and the shape of the aperture cannot be maintained due to the stress made by the mask holder.
Further in the organic EL display unit having the cathode lines formed by the blocking of the partition wall having the reversed taper, the dark spots of the pixels are generated and grown due to moisture in the resist, thereby producing defective display units.
In order to fabricate the full-color organic EL display unit having less defection and a higher definition in the order of hundred micrometers by using the vacuum evaporation, the cathode lines are efficiently deposited through the apertures of the metal mask having the higher minuteness. For realizing the fabrication of the above full-color organic EL display unit, the metal mask should be used having the aperture an accuracy of which is not affected by the tension or the radiation heat.
In view of the foregoing, an object of the present invention is to provide an organic EL device having pixels of higher minuteness and a higher ratio of apertures and a method for fabricating the same. The higher minuteness and the higher ratio of apertures can be achieved by formation of the minute cathode lines having a pixel pitch of about one-hundred xcexcm by using a metal mask (patterning mask).
The present invention provides, in a first aspect thereof, an organic electroluminescent (EL) device including: a substrate; a pair of electrodes each including a plurality of having electrode lines and overlying the substrate; and an organic film including at least one light emitting layer disposed between the electrodes; each of the electrode lines being formed by a plurality of electrode units each overlapped with adjacent electrode units.
The present invention provides, in a second aspect thereof, a method for fabricating an organic EL device including the steps of: forming an organic film sandwiched by a pair of electrodes overlying a substrate; patterning the electrodes to form a plurality of electrode lines, at least one of the electrodes being patterned by moving a patterning mask parallel to the substrate to form a plurality of electrodes units which are overlapped among one another overlying the organic film.
In accordance with the present invention, the minute electrode lines in the order of a hundred micrometer pitch can be formed by, for example, vacuum evaporation by use of the patterning mask (metal mask). When the method for fabrication is applied to an organic EL display unit, the full-color organic EL display unit can be realized with the minuteness of about 254 ppi (pixel per inch).
The above and other objects, features and advantages of the present invention will be more apparent from the following description.